1. Field of the Invention
The present invention relates to bitmap-based digital color printing in which black objects and color objects are intermixedly printed, and more particularly, in which the non-black components of a black color are modified with a process which is responsive to the amount of non-black colorant already in the background color, creates a neutral color when mixed, and may be limited by a preset total colorant maximum.
2. Description of the Related Art
Bitmap-based digital color printers form a bitmap for each of several separations, such as cyan, magenta, yellow, and black. Each bitmap--one bitmap for each color separation--contains a 1 or a 0 for each point on the page which the print head of the color printer is able to address. By this method, the print head of the printer is instructed to turn on or off at each addressable point on the page. The bitmap for each color separation separately commands the print head that is assigned to print that particular color separation. Often, a full-color bitmap-based digital printer will construct a bitmap for cyan, magenta, yellow, and black, and then print each bitmap overlaid on the same page to produce a multitude of perceived colors through the combination of differing amounts of cyan, magenta, yellow, and black.
In particular, the color black may be formed of equal or nearly equal combinations of cyan, magenta, and yellow. In practice, the black colorant is also often added to the combination of cyan, magenta, and yellow colorants to increase the maximum density and avoid problems of unwanted color. When black is formed with a mixture of some of cyan, magenta, yellow, and black, it is known as process black. The color black may also be formed by using the black colorant alone. In this case, it is known as single-color black.
Each type of black, process black and single-color black, have advantages and disadvantages when used in different contexts in the printing process. When single-color black is used over a color field, it is often the case that the single-color black colorant has a gloss which is different from the gloss of the surrounding color field, and the gloss difference can be objectionable. Further, the maximum density of the single-color black colorant is often not as great as the maximum density obtainable with process black, and the density difference is particularly noticeable on certain color backgrounds. In addition, the ink coverage or toner pile height of single-color black may be noticeably less than that of the surrounding color background. Finally, the presence of misregistration between the plurality of separations during the printing process, or the presence of other interactions between the separations, can result in objectionable white or light-colored fringing around a single-color black object on a color background. This occurs for example if the single-color black colorant is not printed precisely and without distortion into the hole left at that position when printing the other separations. On the other hand, when process black is used in a white or gray field which does not contain any non-black colorants, slight misregistrations or interactions among the plurality of separations can result in objectionable color fringing at the edges of the object. Furthermore, the use of multiple colorants to form process black can result in blurred edges, thickened objects, and objectionable amounts of ink or toner coverage.
Process black can also cause problems if the same formulation of process black is used against a variety of color backgrounds. For example, a dense process black with a large amount of colorant, which might be suitable for printing over a dense or dark color background, when printed on a light colored background can create pile height or ink coverage differences between the black object and its light colored background. This can result in printing defects such as tactile unevenness or toner deletion in the background color surrounding the black object. A process black with smaller values of C, M, Y may be better suited for light backgrounds, but can show problems of "caving" if its total pile height is less than the height of the surrounding background for darker backgrounds. Further, if the C, M, Y content of a process black is significantly less than that of the color background, the chances for showing light fringes around the process black object due to misregistration is increased.
These advantages and disadvantages of single-color and process black are known in the graphics arts industry, and knowledgeable graphic artists attempt to design pages which utilize the advantages and avoid the disadvantages. However, some situations, such as a black object placed only partially over a color field, or a color background which changes rapidly, or complex text or graphics over a complex color background, make it difficult to avoid the printing problems associated with mixedly printing black and color. Furthermore, many pages not designed by a knowledgeable graphic artist can benefit from a method and apparatus to automatically create a background dependent black image.
U.S. Pat. No. 4,700,399 to Yoshida discloses a color image processing apparatus which has an edge detector and a control unit for controlling a black reproduction quantity for an edge according to an output from the detector. For edges, densities of color signals for yellow, magenta, and cyan are reduced in accordance with the edge quantity to increase black density.
U.S. Pat. No. 4,953,015 to Hayasaki et. al. discloses a method for printing a color image which makes it possible to obtain a higher density black. Black ink is placed first, and then a plurality of other inks such as cyan, magenta, and yellow are allotted according to a color matrix table and are superimposed on at least a portion of the black ink.
U.S. Pat. No. 5,241,396 to Harrington discloses a method for printing a color image which yields dense black images without thickened, blurred edges. A modified black bitmap is produced by eroding the edges on the original black bitmap. The modified black bitmap is used to instruct the printing of at least one other colorant and then the original black bitmap is used to print the black colorant to yield a dense black image.
These methods have in common the attempt to limit the blurring disadvantages of process black on a white background so that it can be used in place of single-color black.
U.S. patent application Ser. No. 08/673541, "Color Bitmap Generation with Background Dependent Black Objects", discloses a bitmap color printing method which modifies the normal bit block transfer method by which objects are merged into a bitmap prior to printing, in order to be able, without prior knowledge of the current content of the bitmap, to change a black color as it is being merged into the bitmap so that its colorant content is responsive to the background color. (The background color may include white). This helps to maximize print quality when intermixedly printing black objects and color objects. However, this method does not address a problem that by mixing the background colors into the black, it is possible to obtain a process black which in some printing systems contains excessive colorant levels that can create other printer defects such as xerographic "tenting".
U.S. patent application Ser. No. 08/760,204, "Color Bitmap Merging of Black Objects Using Separation Maximum Value Clipping", discloses a bitmap color printing method which modifies the normal bit block transfer method to change a black color as it is being merged into the bitmap so that its colorant content is responsive both to the background color and also to a predetermined maximum colorant level per separation. In this way, the colorant levels in the black object are background-dependent but controlled. This helps to maximize print quality when intermixedly printing black objects and color objects by controlling a larger range of printer defects.
Both of these methods disclosed, however, may create additional printer defects on a system in which the black colorant is not sufficiently opaque to cover off-neutral hues. By being responsive to the background color when mixing colorants into the foreground process black, the methods described may create off-neutral hues in the resulting black color. If responsiveness to the background color causes non-neutral combinations of non-black colorants to be mixed into the black color, and the black colorant is insufficiently opaque to cover the non-neutral hue, then objectionably non-neutral black colors may be created.
By contrast, the current invention describes a bitmap color printing method for generating a process black color that is not only responsive to the amount of colorant in the background and to a predefined limit on total colorant, but also ensures that the generated process black color contains a neutral or near-neutral mix of non-black colorants. In the method described in U.S. patent application Ser. No. 08/760,204, the amount of each non-black colorant found in the background is mixed into the process black foreground up to a predetermined limit for each separation. Instead, the current invention proposes to limit the amount of each colorant not only with a preset maximum, but also by taking into account the other colorants present in such a way as to produce an equal amount of each non-black colorant, which will create a neutral or near-neutral mixture. In this way, neutral black colors are achieved while still maintaining the goal of limiting the amount of non-black colorant in a black object and being responsive to the background color in order to improve print quality.